Hospitals and related medical institutions, doctors' offices and various chemical and medical laboratories all require sterilization equipment. Because high pressure steam destroys heat and moisture sensitive medical instruments, the vast majority of such institutions sterilize those instruments with a "cold" sterilization system based on ethylene oxide. Although ethylene oxide is an effective sterilant, it has three primary deficiencies.
First, the highly flammable nature of ethylene oxide requires its combination with chlorofluorocarbons (CFCs). Because CFCs have been shown to be destructive of the ozone layer, worldwide environmental regulations, e.g., the Montreal Protocol, prohibit CFC production in all major industrial nations after Dec. 31, 1995. As a result, the generally used mixture of 120 ethylene oxide and 88% CFC is effectively banned from the market place.
Second, ethylene oxide gas is neurotoxic and has been classified by OSHA as a carcinogenic substance. As a result, the use of ethylene oxide in hospitals will soon be eliminated independently of the CFC problem.
Finally, the natural properties of ethylene oxide that make it an effective sterilant also require that there be a prolonged aeration period (typically 12-18 hours) to eliminate the toxic residue of ethylene oxide on sterilized articles. The extended aeration period makes it impossible to effectively sterilize large numbers of medical instruments rapidly. This is becoming a significant problem particularly in hospitals and similar institutions where medical treatment is becoming more directed towards same-day procedures and ambulatory surgery.
While some alternative methods to the use of ethylene oxide have recently been introduced, including plasma-gas systems based on peracetic acid and on hydrogen peroxide, none provides a truly acceptable alternative to ethylene oxide which is effective, safe and inexpensive.
While ozone gas in general and humidified ozone gas in particular, have been in use in purification for many years, two principle problems have been associated with its use as a medical instrument sterilant, namely, the difficulties encountered in (1) generating ozone of a sufficiently high concentration and humidity to kill microorganisms and (2) maintaining a continuous flow of such humid, high-concentration ozone for a sufficiently long period of time to penetrate the protective bacterial membranes.
Ozone is an unstable substance which easily degrades to oxygen in the presence of heat or in water. Ozone may be generated several ways, the most common of which is by passing an oxygen-containing gas through a zone between two electrodes and generating a corona discharge between the electrodes. One of the electrodes is a high voltage metal electrode and the other is generally a conductor coated on a dielectric such as glass or ceramic. The first electrode is typically grounded at a high voltage transformer ground and the second electrode is generally electrically attached to the high voltage terminal of a high voltage transformer.
The corona discharge splits the natural oxygen molecules into individual, highly active oxygen atoms, which immediately combine with the nearest oxygen molecule to form a trioxygen molecule, i.e., ozone. The reaction is extremely fast and exothermic. The heat of the reaction, the heat generated by the corona discharge and the heat generated within the dielectric by the passage of current contribute directly to the destruction of the newly generated ozone.
Sterilization is enhanced by the use of humidified ozone. The ozone penetrates the membrane of the bacteria readily when the bacteria is coated with a water film which results from the required humidity level. The penetration of ozone requires less energy when it first passes through hydroxide and when it passes through the bacterial membrane when using a water film as a transfer medium. The water film transfer medium acts in the manner of an impedance transformer, whereby only a small amount of the ozone's molecular energy is required to pass through the bacterial membrane.
Prior art ozone sterilizers which used humidified ozone as a sterilant do not consistently achieve levels of greater than 6% ozone by weight in the humidified sterilant due to heat build-up in the generator.
To increase the quantity and outlet ozone concentration, various attempts have been made to increase the level of cooling and thereby the amount of ozone conversion in the generator. These attempts include feeding cold oxygen to the generator as in U.S. Pat. No. 5,002,738; feeding cold oxygen to one of two parallel annular ozone generation zones such that all of the ozonated gas from a first annular zone must pass through a second annular generation zone before leaving the generator as in U.S. Pat. Nos. 5,008,087, 5,169,606 and 2,936,279; and passing oxygen through a helically-shaped dielectric as in U.S. Pat. No. 4,966,666.
In constructing ozone sterilizers in the past, unsuccessful efforts have been made to increase the rate of conversion from oxygen into ozone by attempting to devise methods of preventing the exothermic heat from destroying the generated ozone formed in the generator.
A further method of increasing conversion rates is by making the generators significantly larger in size. However, larger generators require large quantities of power which not only increases costs, but more importantly, results in significantly higher quantities of ozone-destroying heat. Further, a larger size generator would not be practical in many institutions requiring sterilizers.
Accordingly, it would be beneficial to develop an ozone sterilizer and generator that achieve a high conversion rate without significant loss of ozone in either the generator or the humidification procedure.
Further, it would be advantageous to increase the time that oxygen-containing gas used to form ozone resides in the portion of an ozone generation zone that has the highest ionization energy, i.e., nearest the electrodes, to increase the production of ozone. By increasing the production rate, and minimizing the loss due to generator heat and humidification, ozone sterilizers may be made more efficient and practical.
In view of the incipient disappearance of ethylene oxide as a sterilant, as described above, as well as the apparent lack of success of current alternative processes, there is a need in the art, particularly, the medical and scientific fields, for an efficient and effective ozone generator and sterilizer using a sustained flow of very humid, high-concentration ozone in a safe and controlled manner. There is a need in the art for an ozone sterilizer that is inexpensive to operate, has a short sterilization cycle with a high bacteria kill rate and can produce a high concentration of ozone that can be effective in a large as well as a small sterilization apparatus. Finally, there is a need for an apparatus and method for generating ozone at sufficiently high quantities for introduction into a humidifier such that a controlled amount of an effective humidified ozone sterilant is provided to achieve a high bacteria kill rate in as short a time as possible.